There is increasing interest in mapping dimensions of personality and psychopathology onto biological systems. Fear is a key construct in this regard because it represents a dimension of human variation with direct biological significance and clear mental health implications. The broad objective of the proposed research is to explore etiologic contributions to trait fear and fearlessness in human participants and to examine brain mechanisms associated with individual differences in fear reactivity. A foundation for this work is prior research demonstrating impairments in fear-potentiated startle among incarcerated psychopathic individuals, who have been conceptualized as fearless. More recent research has revealed similar impairments in community participants high in Fearless Dominance, a psychopathy-related construct that acts as a low pole indicator of trait fear. Three phases of research are proposed to extend this work, with the following aims in mind: (1) Psychometric measures of trait fear and fearlessness will be collected from a large community twin sample to evaluate genetic and environmental contributions to a latent fear dimension;(2) A subset of these twin pairs will be tested in the laboratory to establish fear-potentiated startle as a marker of this underlying trait fear dimension, and to determine the etiologic basis of this association;and (3) Twin pairs selected to be low or high on both trait-fear scores and level of fear-potentiated startle will be tested in a neuro-imaging (fMRI) study to investigate differences in brain reactivity associated with variations in trait fear, and to yield clues as to the etiology of these differences. This research will contribute to our understanding of the biological underpinnings of trait fear and fearlessness in humans, and in connection with this, advance conceptualization and measurement of a construct of fundamental relevance to personality and psychopathology.